The present invention relates in general to devices for rectifying alternating current signals. More specifically, the invention relates to a full-wave rectifier finding particular utility in low-level envelope type video detection circuits.
Conventionally, a received television signal is in the form of an RF carrier modulated by a video signal. In the television receiver, the modulated RF carrier is typically converted to an intermediate frequency (IF) signal which is filtered and amplified and then applied to a video detector. The video detector demodulates the modulated IF signal to recover the video information in the form of a video signal. The video signal developed by the video detector is subsequently coupled through various signal processing circuits in the receiver and finally to the electron gun structure of the picture tube for producing an observable image corresponding to the video information characterizing the transmitted television signal. The video signal is also coupled to a synchronizing signal separator circuit which develops signals applied to deflection circuitry for controlling the deflection of the electron beam produced by the electron gun structure in synchronism with the horizontal and vertical synchronizing components of the video signal. The output of the video detector is also normally applied to an automatic gain control circuit which controls a variable gain amplifier to maintain the amplitude of the video signal substantially constant.
Prior art video detectors can be conveniently classified into two basic varieties; namely, synchronous detectors and envelope or peak detectors. Synchronous video detectors generally comprise circuitry for multiplying the modulated IF subcarrier signal with a phase and frequency controlled regenerated carrier signal to effect a rectification of the IF subcarrier. The rectified product is subsequently coupled through a suitable low-pass filter for recovering the video information. Examples of prior art synchronous detectors of this type are shown in U.S. Pat. Nos. 3,961,360 to Sato et al, 3,764,925 to Von Nikelsberg et al, 3,871,022 to Wilcox and 3,624,275 to Lunn. Although synchronous detectors of the general type illustrated in these patents possess a number of desirable attributes, including generally excellent low-level signal handling capabilities, various practical considerations have recently been encountered which militate against their use in the roll of a television receiver video detector in certain situations.
Envelope or peak type video detectors conventionally comprise a rectifier, such as a diode, connected for rectifying the modulated IF subcarrier signal and a low-pass filter for removing the subcarrier signal from the half-wave rectified output developed by the diode. Sometimes, a plurality of diodes are used to full-wave rectify the modulated IF signal before coupling it to the low-pass filter. In either event (i.e., full-wave or half-wave rectification), however, the modulated IF subcarrier signal must reach a level of about 0.6-0.7 volt to render the rectifying diode conductive. Stated otherwise, for input signal levels less than about 0.6-0.7 volts the rectifying diode is non-conductive thereby introducing a degree of distortion into the rectified output signal. The effect of this distortion is normally minimized by driving the diode with a relatively large input signal, generally about 3 volts, compared to the 0.6 volts necessary to cause the diode to conduct. While this practice has heretofore been considered acceptable, as more integrated circuitry is used in the design of electronic equipment and in the design of television receivers in particular, the task of providing such relatively high level input signals has rendered the use of conventional diode rectifiers unattractive.
U.S. Pat. No. 4,069,460 to Sauer discloses a current comparator circuit which, if used as a rectifier, may overcome some of the foregoing problems in the prior art. The circuit disclosed in this reference is, however, relatively inflexible and not readily modifiable to provide increased linearity in situations where it is desired to precisely demodulate an input signal. In particular, the rectified output is produced at the collectors of a pair of switching transistors whose states are controlled by circuitry characterized by a pre-established error which may not be conveniently reduced in situations where a more nearly linear operating characteristic is desired.
It is, therefore, a basic object of the present invention to provide an improved rectifier circuit capable of operating in response to relatively low-level input signals.
It is a more specific object of the invention to provide an improved full-wave rectifier circuit useful in association with a low-pass filter for providing a low-level envelope type video detector.